Molded case circuit breakers are currently available for operating over a wide range of currents. Over such a wide range of operating currents, the circuit breaker operating mechanism design remains somewhat unchanged, while the current carrying components must be increased to be able to handle the increased ampacity. One of the most rigid tests that must be performed before a breaker acquires the requisite approvals, is a short circuit withstand test, wherein the breaker is subjected to a short circuit current up to several hundred times the current that the breaker is rated for under steady state conditions. It has become more difficult to increase the size of the breaker components without increasing the size of the breaker case dimensions.
One of the factors causing breaker failure under short circuit test conditions is damage from the arc blast which occurs during contact arcing. The blast extends from the arc chamber to completely engulf the breaker operating mechanism. The high temperatures existing within the gases evolved during arcing severely affect the temper of the operating spring as well as degrading the properties of the contact arm and the latch assembly. At very high short circuit current conditions, exposure of the operating mechanism to the intense arc blast can eventually cause the operating mechanism to fail.
Several earlier attempts were made to protect the operating mechanism from the arc blast which occurs during contact arcing.
U.S. patent application Ser. No. 353,449 filed Mar. 1, 1982, describes a thermal shield for protecting the operating spring during contact arcing conditions.
U.S. Pat. No. 4,011,420 entitled "Molded Case Circuit Breaker With Improved Interrupting Capacity" describes a shield which is formed by projections extending from the circuit breaker cover and base assembly.
U.S. Pat. Nos. 3,268,689 and 3,287,522 describe circuit breaker contact assemblies having dual parallel blade contact arms wherein the contact arm and flexible conductor are able to withstand a number of mechanical operations without damage.
One method of protecting the circuit breaker operating mechanism from the arc blast consisted of a movable arc shield which was attached at one end to the movable contact arm. The function of the movable arc shield was to protect the contact operating spring from the arc blast during contact arcing. The arc shield of the instant invention improves over the movable arc shield of the prior art since the movable arc shield could become separated from the contact arm and become inoperative.
The purpose of this invention is to provide a unique arc shield arrangement, when used in combination with the aformentioned parallel blade contact arm assembly, results in a circuit breaker having substantially improved short circuit current withstand.